Lockstitch loop takers for sewing machines

ABSTRACT

A lockstitch, rotary hook including a stationary bobbin case with a bobbin thread tensioning spring located diametrically opposite the place of loop seizure of the hook and with a passageway for the bobbin thread to the stitching point extending between the bobbin case and the rotary hook and being subjected to periodic bobbin thread pull-off action of relatively moving surfaces on the bobbin case and the rotary hook.

[15] 3,693,565 51 Sept. 26, 1972 .lohnsonum................112/184 Ketterer 12/184 LOCKSTITCH LOOP TAKERS FOR SEWING MACHINES [72] lnventor:

St I .Ktt J b ,NJ. J e erer ames mg Primary ExammerH. Hampton Hunter Asslgneei The 08 p y, New York Attorney-Marshall J. Breen, Chester A. Williams, Jr.

and Robert E. Smith [57] ABSTRACT A lockstitch, rotary hook including a stationary bob- [22] Filed: April 20, 1971 [21] Appl. N0.: 135,671

' bin case with a bobbin thread tensioning spring [52] US. located diametricany opposite the place of loop 112/184 Int. 57/08 112/184, 181, 182,183,189,

seizure of the hook and with a passageway for the bobbin thread to the stitching point extending between the bobbin case and the rotary hook and [58] Field of Search......

being subjected to periodic bobbin thread pull-off ac- References Cited UNITED STATES PATENTS tion of relatively moving surfaces on the bobbin case and the rotary hook.

2,862,468 12/1958 Johns0n.................112/184 X 9 Claims, 7 Drawing Figures PATiNTinsirzslsn 3.693.565

SHEET 1 BF 3 x INVENTOR.

Stanley J. Ketterer wnmass:

dw- MM/e1 ATTORNEY PATENTEDSEP26 I972 sum 2 or 3 INVENTOR. Stanley J. Keherer BY w Xjwz ce 71 ATTOR N EY PATENTEDSEPZS 1922 SHEET 3 0F 3 INVENTOR.

Stanley J. Ketterer BY W411 ATTORNEY LOOP TAKERS FOR sswmc MACHINES BACKGROUND OF THE INVENTION In sewing machines capable of sewing both zigzag and straight stitches, the critical requirements for these LOCKSTITCH two different types of sewing have heretofore been difficult to reconcile. For sewing zigzag stitches it is desirable that the stitches on both'sides of the centerfavor one over the other.

For straight stitching, however, it becomes important that the threads of each stitch be concatenated alike so that each successive stitch will have uniform appearance. If the needle does not always penetrate on the same side of the bobbin thread, the stitches will not be concatenated alike; penetration on one side of the bobbin thread resulting in a proper lockstitch, and penetration on the opposite side resulting in what is termed a hitch stitch. A hitch stitch is not only different in appearance and thus objectionably noticeable in a seam but, if repeated sufficiently, can cause untwisting and breakage of the needle thread. To avoid hitch stitching when sewing straight stitches, therefore, a bobbin thread lead from one side of the line of stitches is desirable.

These differing requirements for bobbin thread lead are seemingly incompatible. Prior loop takers have settled upon various compromises in bobbin thread lead, and it is known to provide a bobbin thread lead from one side for good straight stitching together with a bobbin thread pull-off device to reduce the disadvantage of this bobbin thread lead on the balance when sewing zigzag stitches.

SUMMARY OF THE INVENTION In the present invention, the bobbin thread is directed to the stitching point through guide means preferably including a thread tensioning spring located on the bobbin case diametrically opposite the place of loop seizure of the loop taker. The thread guide is located in substantial alignment with the direction in which the work is fed between stitches, and the distance from the thread guide ,to the stitching point is greater than inmost prior stitching mechanisms. For both of these reasons and particularly with rotary hooks arranged on an axis parallel to the axis of needle reciprocation, this arrangement provides for an ideal balance between the right and left-hand zigzag stitches. The bobbin thread, moreover, is passed between the loop taker and the bobbin case as it is directed to the stitching point, and is thus subjected to a wiping action to one side. Means are provided for varying this wiping or pull-off action so that at needle penetration the bobbin thread lead will consistently occur from the same side of the needle, thus during straight stitching obviating the formation of hitch stitches.

With the above and additional objects and advantages in view, as will hereinafter appear, this invention will now be described with reference to the accompanying'drawings of a preferred embodiment in which:

FIG. 1 represents a vertical cross sectional view taken lengthwise along a sewing machine bed and through the axis of rotation of a rotary hook constructed in accordance with this invention,

FIG. 2 is an elevational view of the bobbin case, illustrating the bobbin thread tensioning spring means with bobbin thread issuing therefrom as during sewing,

FIG. 3 is a disassembled perspective view of the rotary hook of this invention including the mechanism for constraining the bobbin case in place in the rotary hook,

FIG. 4 is a top plan view of the rotary hook ofFIGS. l and 3 together with the mechanism for constraining the bobbin case in place in the rotary hook and including the bobbin thread shown at stitch setting time,

FIG. 5 is a top plan view of'the rotary hook of FIG. 4 together with only a fragment of the bobbin case showing the bobbin thread tensioning spring means of the bobbin case and indicating the path of the bobbin thread therefrom when a stitch is being set into the work fabric, I

FIG. 6 is a top plan view of the rotary hook similar to FIG. 5 but showing the path of the bobbin thread at the time of needle penetration of the work, and

FIG. 7 is an enlarged cross sectional view of the rotary hook and the sewing threads showing the bobbin thread pull-off flange of the bobbin case taken substantially along line 77 of FIG.6, and including fragments of the needle and throat plate.

The loop taker illustrated in the drawings is adapte for use with a sewing machine such as is disclosed in the US. Pat. No. 2,862,468, Dec. 2, 1958, of R. E. Johnson which is incorporated herein by reference. The abovecited patent may be referred to for disclosure of all of the sewing machine mechanism save for the loop taker and mechanism directly related with the loop taker such as the bobbin thread replenishing mechanism.

In the drawings, 20 denotes the bed of a sewing machine frame carrying a bushing 21 in which a bed shaft 22 is journalled. The bed 20 is formed with an up wardly. open loop taker accommodating cavity 23 closed by a slide cover plate 24 and by a throat plate 25. Rotatable in a bushing 26 carried in the bed is a hollow loop taker shaft 27 which is attached as by welding or the like to the underside of a loop taker indicated generally at 28 which is disposed in the cavity 23.

The loop taker disclosed in the drawings is a rotary hook which is imparted turning movement in one direction during operation of the sewing machine by a bevel gear 29 made fast on the loop taker shaft by a set screw 30 and in mesh with a bevel gear 31 secured by a set screw 32 to the bed shaft. Preferably the rotary hook partakes of two revolutions during each cycle of endwise reciprocation of a needle 33 which cooperates therewith in the formation of stitches passing through a needle hole 34 in the throat plate.

The rotary hook of this invention has an upwardly open cup-shaped form including at the rim an upwardly extending bearing shoulder 39. The rim is also formed with an annular inwardly extending bearing rib 40 having a lateral opening 41 formed at one side with a needle thread loop seizing beak 42.

The cup-shaped rotary hook 28 includes a base 43 formed internally with an annular recess 44, a portion of which may extend completely through the hook base to provide clearance for the dip of the needle. The recess 44 has another function, i.e., to isolate a raised annular boss 45 at the center of the base. A large segment of the edge of the boss 45 is chamfered as at 46 and a small segment is chamfered as at 47, the intervening portions 48 and 49 of the peripheral edge being substantially square. This raised boss 45 serves an important function in pulling off and positioning a bobbin thread during sewing in which regard the specific form, position and cooperative relation with other elements will be described in detail hereinbelow.

Constrained within the cup-shaped rotary hook is a bobbin case indicated generally at 50. The bobbin case is formed with an upwardly open bobbin accommodating cavity 51 through the bottom of which a large control aperture 52 extends. Externally, the bobbin case is formed with a bearing flange 53 which rests upon the bearing rib 40 of the rotary hook and is constrained radially against the bearing shoulder 39. The bobbin case flange 53 at one side is slabbed as at 54 substantially along a chord to provide clearance for the path of needle reciprocation between the bobbin case and the rotary hook. Adjacent to the slabbed portion 54 the bobbin case flange 53 is also formed with a shallow recess 55 into which fits a rotation restraining element 56 secured by a screw 57 to the machine bed 20. The bobbin case flange 53 substantially diametrically opposite the slabbed portion 54 is formed with a recess 58 on which a support arm 59 of a thread tensioning spring bracket 60 is secured by a fastening screw 61.

The bobbin case 50 adjacent to the recess 58 in the flange 53 is formed with a radial opening 80 across which opening the thread tensioning spring bracket 60 spans. A downtumed arm 81 on the bracket 60 is also disposed in the bobbin case radial opening 80 and provides a mounting surface against which a bobbin thread tensioning spring 82 is secured by a screw 83. Preferably the spring 82 is formed with slot 84 in the form of a U-shaped loop separating a tongue 85 through which the securing screw 83 and a locating pin 86 on the bracket arm 81 pass. Surrounding the tongue 85 is an outer spring blade portion 87 which frictionally engages the bobbin thread against the downtumed bracket arm 81 to provide the bobbin thread tension. The downtumed arm 81 is formed with a thread entry slot 88 which, as shown in FIGS. 2 and 3, terminates in an eyelet 89 for directing the thread to the spring blade portion 87 of the spring. Below the spring 82, the downtumed arm 81 is formed with a thread guiding groove 90 from which the bobbin thread leaves the spring and proceeds to the stitching point. The tension exerted on the thread by the spring blade portion 87 may be adjusted by regulating the fastening screw 83.

For additional restraint of the bobbin case within the rotary hook, a holddown spring arm 62 which extends from a base plate 63 and a rotation restraining arm 64 extending from a base plate 65 are provided. Each of the base plates 63 and 65 is formed with an aperture 66 for a shouldered screw 67 threaded into the machine bed; and each of the base plates is formed with an elongated slot 68 to accommodate the eccentric head 69 of a locating pin 70 secured by a set screw 71 in the machine bed. The rotation restraining arm 64 is formed with a downtumed lip 72 which abuts a shouldered portion 73 on the support arm 59 of the thread tensioning spring bracket 60.

The base plate 65 is formed with another arm 75 which, as shown in FIGS. 4 and 7, extends alongside the slabbed portion 54 of the bobbin case between the path of needle reciprocation and the bobbin case. This arm 75, as will be explained in greater detail hereinbelow, acts as a support for the bobbin thread during sewing to hold the bobbin thread away from the path of the loop taker beak 42. The arm 75 is formed with a downwardly tapering upper edge 76 joined at the free extremity by an upwardly projecting tang 77. The purpose of the thread tang 77, is to prevent a bobbin thread from being carried off the free end of the arm 75. Also formed at the free extremity of the arm 75 is an outwardly extending thread guard finger 78 adapted to project beneath the inwardly extending bearing rib 40 on the rotary hook when the base plate 65 is secured in place. The thread guard finger 78 serves to direct each needle thread loop manipulated by the rotary hook beak completely over the arm 75 and thus prevents the needle thread loops from lassoing the arm 75. It is also the purpose of the outwardly extended finger 78 to prevent the beginning end of a bobbin thread from accidentally being directed twice to the bobbin thread tensioning spring when a prewound bobbin is inserted into the machine.

Secured in a reception seat formed beneath the bobbin case at the side opposite the thread tensioning spring 82 is an arcuate thread camming blade 91 preferably having a curvature coinciding closely with that of the boss 45 which projects upwardly within the cup-shaped rotary hook. As shown in FIG. 7 the blade 91 depends below the level of the boss 45 and is spaced outwardly thereof to provide thread clearance. A notch 92 is formed near one extremity of the blade 91 to limit the influence which the blade can exert on the bobbin thread as will be described below.

Freely rotatable in the bobbin accommodating cavity 51 of the bobbin case is a bobbin indicated generally at 95. The bobbin includes a top flange 96 formed with an outwardly extending slot 97 skewed slightly from a true radial position on the top flange. The bobbin is formed with a bottom flange 98 having a depending boss 99 which fits into the central aperture 52 of the bobbin case. The boss 99 is formed with a noncircular central aperture 100. As shown in the drawings, the aperture 100 may be hexagonal to match the hexagonal extremity 101 of a driving spindle 102 which is arranged in the hollow loop taker shaft for driving the bobbin when it is desired to replenish thread thereon. Between the flanges 96 and 98 of the bobbin a plain cylindrical hub 103 is provided.

The spindle 102 is part of a bobbin thread replenishing mechanism indicated generally at in the drawings. Since this replenishing mechanism does not form a direct part of this invention and this rotary hook is operative and advantageous without a bobbin thread replenishing mechanism, the replenishing mechanism will not be described in further detail herein. A more detailed description may be had in my copending US. patent application Ser. No. 136,929 filed Apr. 23, 197 l to which reference may be had.

In order to ready the rotary hook of this invention for sewing, the bobbin 95 with thread B wrapped on the hub 103 is placed in the bobbin cavity 51 of the bobbin case 50 and the bobbin thread B is directed between the thread tensioning spring bracket 60 and the bobbin case, into the thread guide slot 88 to the eyelet 89 and between the bracket arm 81 and the thread tensioning spring blade 87 to the groove 90 as shown in FIG. 2. When a loop of needle thread N is seized by the hook beak as shown in FIG. 4, manipulated completely about the bobbin case, and drawn up through the throat plate aperture 34, the bobbin thread B will be drawn by this needle thread loop beneath the bobbin case and will traverse the boss 45 of the rotary hook as shown in FIGS. 5,6 and 7.

Referring to FIGS. 5, 6 and 7, it will be observed that the bobbin thread B, when it extends between the bobbin case and the rotary hook is deflected by the blade 91' depending from the bobbin case. FIGS. 6 and 7 illustrate the position of parts as the chamfer 47 moves out of engagement with the thread. When the bobbin thread traverses either chamfer 47 or 46, a relatively direct path for the bobbin thread is provided about the depending edge of the blade 91. When the bobbin thread traverses the substantially square edge portions 48 and 49 of the boss 45, however, a more tortuous path must be assumed by the bobbin thread. The frictional resistance on the thread, which the relative movement of the rotary hook boss 45 and the blade 91 impose, is thus far greater when the thread must traverse a square edge 48 or 49 of the boss 45 than it is when the thread traverses a chamfer 46 or 47. As a result, the bobbin thread B will be wiped to the right as viewed in FIG. 6 when traversing a square edge, while this wiping action will be greatly reduced when the thread traverses a chamfer. While traversing a chamfer, therefore, the bobbin thread is freed and may be drawn almost diametrically across the rotary hook as shown in FIG. 5. When traversing a square edge, the bobbin thread will be wiped to the left as shown in FIG. 6, but never completely off the blade 91 because of the detaining action of the notch 92.

Preferably, the large chamfer 46 is timed to traverse the bobbin thread B while the needle is being raised and the threads of the stitch are being drawn up into the work. The bobbin thread B will, therefore, have substantially equal influence on the formation of a lefthand and a right-hand zigzag stitch. In FIG. 5, with the rotary hook turning in a counterclockwise direction in which the beak 42 faces, the chamfer 46 is moving out of engagement with the bobbin thread B and the square edge portion 48 is moving into engagement with the thread so that a bobbin thread wiping action will begin as the stitch as being finally set into the work.

Bearing in mind that the rotary hook partakes of two revolutions for each needle reciprocation, the beak 42 in FIG. 5 is about to begin an idle revolution as the wiping or pull-off of the bobbin thread begins.

In FIG. 6, the hook has rotated approximately 140 beyond the position shown in FIG. 5. The chamfer portion 47 will have interrupted the pull-off action on the bobbin thread and the square edge portion 49 is beginning to reapply the wiping pull-off action as the needle penetrates the work. It has been found that a plurality of successive square edge portions are more effective as a pull-off than one long square edge portion, and therefore, the short chamfer portion 47 provides for a relaxation of the wiping action on the thread so that the square edge portion 49 will be freshly effective and exerting a full effective thread positioning effect on the bobbin thread when the needle penetrates the work as illustrated in FIG. 6. In this way, during straight stitching, the needle will pass always to the right of the bobbin thread as viewed in FIG. 6 and the formation of hitch stitches will be prevented.

The loop taker beak 42, as is conventional in the art, seized a loop of needle thread as the needle rises and, therefore, both limbs of each seized needle thread loop will extend from the beak upwardly toward the needle aperture 34 in the throat plate. Both limbs of each seized needle thread loop will, therefore, pass above the outturned finger 78 of the thread supporting arm 75. Regardless of any other manner in which the needle thread loop is manipulated, it will pass in its entirety over the thread supporting arm 75.

Each needle thread loop seized by the loop taker is spread by the loop taker and passed completely about the bobbin case 50 as is known in the art of lockstitch formation. Any bobbin thread extending from the bobbin case will thus be encompassed by the needle thread loop and drawn by it upwardly through the needle aperture 34 in the throat plate. The bobbin thread will thus be drawn by the needle thread over the thread supporting arm as shown in FIG. 7.

The thread supporting arm 75 serves an important function in maintaining the bobbin thread or any of the sewing threads once concatenated into stitches from dropping into the path of the loop taker beak which would invariably cause thread breakage. The danger of the bobbin thread or the threads concatenated into stitches dropping into the path of the loop taker beak can arise whenever these threads are abnormally slackened. This may occur, for instance, when sewing is continued beyond the edge of a fabric panel. The socalled chain of stitches, which may be formed beyond a fabric panel or between successive fabric pieces being stitched, is difficult to control and may very readily be slackened causing either the entire chain or, more prevalently, the bobbin thread to shift toward the loop taker beak path. The thread supporting arm 75 when it is beneath these threads deters this from occurring.

The loop taker of this invention is organized to include means for replenishing thread on the bobbin while in place in the bobbin case by directing needle thread thereto. While this bobbin thread replenishing mechanism does not form a direct part of the present invention and will not be described in detail herein, it does require that a path must be possible for the needle thread to be admitted to the bobbin thread tensioning spring 82. This path for admittance of needle thread occurs between one edge 120 of the radial opening 80 in the bobbin case and the leading edge 121 of the thread tensioning bracket 60. For the purposes of the present invention, provision must be made to prevent threads from being admitted to the bobbin thread tensioning spring during normal sewing operations.

The needle thread loops seized by the loop taker beak will not be admitted to the passageway between the edges and 121 because the needle thread limbs proceeding from the needle aperture 34 in the throat plate 25 will extend substantially perpendicularly across the edges 120 and 121.

In the event that an operator should place a prewound bobbin into. the bobbin case, manually threading the bobbin thread to the action of the spring 82 with the free end of bobbin thread proceeding from the groove 90, it is possible that, before a needle thread loop can encompass the free end of bobbin thread to draw it upwardly through the needle aperture,.the free end of bobbin thread may become seized by. the loop taker beak. If seized by the beak 42, the bobbin thread extremity would extendv across the bobbin case not from the needle aperture 34 as in the case of a needle thread, but rather, from the vicinity of the groove 90 in which case the seized bobbin thread loop would be shed from the beak 42 in a position with the upper limb of the bobbin thread loop disposed substantially parallel to the edges 120 and 121 defining the passageway to the bobbin thread tensioning spring. Admission of the bobbin thread twice to the spring 82 would cause such a high bobbin thread tension as to render the formation of stitches impossible. As a deterrent to such a condition is the second function of the finger 78. on the thread supporting arm 75. Any free extremity of bobbin thread which might be picked up by the beak 42 of the rotary hook will not simply slip around the bobbin case but will be detained on The finger 78. As a result, any seized bobbin thread will be carried by the beak substantially in the same manner as any seized needle thread loop. The free end of the seized bobbin thread will pass substantially perpendicular to the edges 120 and 121 and the seized bobbin thread will be carried by the beak well beyond the edges 120 and 121 before his shed thus preventing the bobbin thread from entering the passageway to the tensioning spring.

Having set forth the nature of this invention, what is claimed herein is:

1. A lockstitch forming loop taker for a sewing machine having a thread carrying needle which is endwise reciprocable in a path cooperating with the loop taker through a needle aperture in a work-supporting plate, said loop taker comprising a circularly moving loop taker body formed with an annular raceway and a needle thread loop seizing beak, a bobbin case journailed in said annular raceway and formed with a bobbin accommodating cavity, means for constraining said bobbin case stationary with respect to said circularly moving loop taker, thread guiding means on said bobbin case located opposite the point of seizure of said needle loop by said loop taker beak for directing a bobbin thread from said bobbin accommodating cavity, said thread guiding means being so arranged on said bobbin case that the path of the thread from said thread guiding means to said needle aperture in the work-supporting plate extends between said stationary bobbin case and said circularly moving loop taker body, and said loop taker body being formed with bobbin thread manipulating surfaces for engagement with a bobbin thread in said path between said bobbin case and said loop taker body.

2. A lockstitch forming loop taker as set forth in claim 1 in which said bobbin thread manipulating surfaces on said loop taker body comprises a raised annular-boss formed within said loop taker body, an arcuate thread camming blade carried on said bobbin case closely adjacent to said raised annular boss when said bobbin case is constrained in said loop taker, said raised annular boss being provided with alternate square and chamfered edge portions to vary the influence of the loop taker on the bobbin thread.

3. A lockstitch forming loop taker as set forth in claim 2 in which said square and chamfered edge portions of said raised annular loss on said loop taker body are positioned relatively to said loop seizing beak so as to bring a square edge portion into engagement with the bobbin thread during penetration of said needle through said needle aperture, and to bring a chamfered edge portion into engagement with the bobbin thread during the setting of each lockstitch.

4. A lockstitch forming loop taker as set forth in claim 1 having the axis of circular movement of the loop taker substantially parallel with the path of endwise reciprocation of the needle, and in which a stationary bobbin thread guard is provided between the path of needle reciprocation and the bobbin case, said bobbin thread guard having a free extremity facing in the direction of movement said loop seizing beak past the path of needle reciprocation and with the bobbin case defining a passageway at the side opposite the path of needle reciprocation for egress of the bobbin thread from between the loop taker body and the bobbin case.

5. A lockstitch forming loop taker as set forth in claim 4 in which the top surface of said bobbin thread guard adjacent the path of needle reciprocation slopes in a direction away from the work-supporting plate toward the free extremity of the bobbin thread guard, and in which the free extremity of the bobbin thread guard is arranged to one side of the path of needle reciprocation and is formed with a thread detaining projection extending toward the work-supporting plate.

6. A rotary hook assembly for a lockstitch sewing machine includinga circularly movable hook body formed with a loop seizing beak a bobbin accommodating case journalled in said hook body, means for constraining said bobbin case stationary relatively to said hook body, and said bobbin case being formed at one side with an opening to provide clearance for the path of reciprocation of a sewing machine needle, in which the improvement comprises a thread guide means on said bobbin case for egress of a bobbin thread therefrom, said thread guide means being located on said bobbin case substantially diametrically opposite said needle path clearance opening to provide for an attentuated thread path between the bobbin case and the hook body for said bobbin thread extending from the thread guide means to the stitches being formed.

7. A rotary hook assembly for a lockstitch sewing machine as set forth in claim 6 in which said bobbin case opening to provide clearance for the path of reciprocation of a sewing machine needle is laterally elongated to accommodate lateral shift of the needle path during zigzag sewing, and in which said thread guide means is located substantially equidistant from opposite extreme lateral positions of the needle path during zigzag stitching.

8. A rotary hook assembly for a lockstitch sewing machine as set forth in claim 6 in which bobbin thread manipulating means is carried on said hook body for operative engagement with said bobbin thread traversing the attentuated thread path between the bobbin case and the hook body. 

1. A lockstitch forming loop taker for a sewing machine having a thread carrying needle which is endwise reciprocable in a path cooperating with the loop taker through a needle aperture in a work-supporting plate, said loop taker comprising a circularly moving loop taker body formed with an annular raceway and a needle thread loop seizing beak, a bobbin case journalled in said annular raceway and formed with a bobbin accommodating cavity, means for constraining said bobbin case stationary with respect to said circularly moving loop taker, thread guiding means on said bobbin case located opposite the point of seizure of said needle loop by said loop taker beak for directing a bobbin thread from said bobbin accommodating cavity, said thread guiding means being so arranged on said bobbin case that the path of the thread from said thread guiding means to said needle aperture in the work-supporting plate extends between said stationary bobbin case and said circularly moving loop taker body, and said loop taker body being formed with bobbin thread Manipulating surfaces for engagement with a bobbin thread in said path between said bobbin case and said loop taker body.
 2. A lockstitch forming loop taker as set forth in claim 1 in which said bobbin thread manipulating surfaces on said loop taker body comprises a raised annular boss formed within said loop taker body, an arcuate thread camming blade carried on said bobbin case closely adjacent to said raised annular boss when said bobbin case is constrained in said loop taker, said raised annular boss being provided with alternate square and chamfered edge portions to vary the influence of the loop taker on the bobbin thread.
 3. A lockstitch forming loop taker as set forth in claim 2 in which said square and chamfered edge portions of said raised annular loss on said loop taker body are positioned relatively to said loop seizing beak so as to bring a square edge portion into engagement with the bobbin thread during penetration of said needle through said needle aperture, and to bring a chamfered edge portion into engagement with the bobbin thread during the setting of each lockstitch.
 4. A lockstitch forming loop taker as set forth in claim 1 having the axis of circular movement of the loop taker substantially parallel with the path of endwise reciprocation of the needle, and in which a stationary bobbin thread guard is provided between the path of needle reciprocation and the bobbin case, said bobbin thread guard having a free extremity facing in the direction of movement said loop seizing beak past the path of needle reciprocation and with the bobbin case defining a passageway at the side opposite the path of needle reciprocation for egress of the bobbin thread from between the loop taker body and the bobbin case.
 5. A lockstitch forming loop taker as set forth in claim 4 in which the top surface of said bobbin thread guard adjacent the path of needle reciprocation slopes in a direction away from the work-supporting plate toward the free extremity of the bobbin thread guard, and in which the free extremity of the bobbin thread guard is arranged to one side of the path of needle reciprocation and is formed with a thread detaining projection extending toward the work-supporting plate.
 6. A rotary hook assembly for a lockstitch sewing machine including a circularly movable hook body formed with a loop seizing beak a bobbin accommodating case journalled in said hook body, means for constraining said bobbin case stationary relatively to said hook body, and said bobbin case being formed at one side with an opening to provide clearance for the path of reciprocation of a sewing machine needle, in which the improvement comprises a thread guide means on said bobbin case for egress of a bobbin thread therefrom, said thread guide means being located on said bobbin case substantially diametrically opposite said needle path clearance opening to provide for an attentuated thread path between the bobbin case and the hook body for said bobbin thread extending from the thread guide means to the stitches being formed.
 7. A rotary hook assembly for a lockstitch sewing machine as set forth in claim 6 in which said bobbin case opening to provide clearance for the path of reciprocation of a sewing machine needle is laterally elongated to accommodate lateral shift of the needle path during zigzag sewing, and in which said thread guide means is located substantially equidistant from opposite extreme lateral positions of the needle path during zigzag stitching.
 8. A rotary hook assembly for a lockstitch sewing machine as set forth in claim 6 in which bobbin thread manipulating means is carried on said hook body for operative engagement with said bobbin thread traversing the attentuated thread path between the bobbin case and the hook body.
 9. A rotary hook assembly for a lockstitch sewing machine as set forth in claim 8 in which said bobbin thread manipulating means is operative to engage and shift the bobbin thread laterally during the downstroke of the needle toward the bobbin case clearance opening and is effective to release the bobbin thread during the upstroke of the needle. 